1. (Field of the Invention)
This invention relates to a CT image processing apparatus having a magnetic disk storage device in the medical diagnostic field.
2. (Description of the Prior Art)
In a CT (computerized tomography) apparatus, reconstruction image data is stored in a storage device such as a magnetic disk storage device (simply referred to as "a magnetic device") with or without being compressed to a smaller data amount.
In general, when the amount of reconstruction image is expressed in 16 bits, a large capacity of 200 Kbytes is required for a 320.times.320 matrix.
For headscans for a patient, 10 to 20 CT images are acquired. Therefore, the amount of data acquired by the apparatus per day is therefore about several ten Mbytes.
Compression of the image data can be performed by various methods. According to one method, first image data A0 stored in a main memory is stored in a first area of the memory disk. As for second image data A1 in the main memory, the difference of two pixels or difference data B1 between the first image data A0 and second image data A1, i.e., (A1-A0) is stored in a second area of the memory disk. As for third image data A2 in the main memory, difference data B2 between the second image data A1 and third image data A2 is stored in a third area of the memory disk. Likewise, the differences of the every adjacent two image data are stored in predetermined areas of the memory disk. When the image data in the main memory has 16 bits and the difference between every two adjacent image data has 8 bits, compression of the image data to a data length of 8 bits is performed. In this compression method, when the difference between two adjacent image data exceeds 127 (i.e., 2.sup.7 -1=127), a flag S of 8 bits representing an overflow is stored in an area preceding the area for storing this difference.
Registration and storage of image data according to the non-compression method has an advantage in that image data can be read out from the memory disk and be transferred to and displayed at a display device without performing any data conversion. This means that fast access to the image data can be performed. However, since the amount of one image data is relatively large, only a small number of image data can be stored in the memory disk.
Registration and storage of image data according to the compression method has an advantage in that a large number of image data can be stored in a memory disk since the amount of each image data is small due to data compression. However, when image data is read out from the memory disk, the image data must be subjected to expansion processing. In other words, as shown in FIG. 2, the second difference data B1 stored in the second area of the memory disk is added to the first image data A0 stored in the first area of the memory disk, and the obtained second image data A1 is stored in the main memory. Therefore, the access time of image data is prolonged by a time required for such expansion processing.
For example, when a sagittal image of a specific portion of the patient is to be obtained from the acquired image data, lengthy processing must be performed.
The present invention has been made in consideration of this and has as its object to provide a CT image processing apparatus in which both compressed and non-compressed image data are stored in a memory disk, and a high-speed access of image data can be performed without reducing the permissible number of image data to be stored.